Merge #9814

9814: Generate default impl when converting `#[derive(Debug)]` to manual impl r=yoshuawuyts a=yoshuawuyts

This patch makes it so when you convert `#[derive(Debug)]` to a manual impl, a default body is provided that's equivalent to the original output of `#[derive(Debug)]`. This should make it drastically easier to write custom `Debug` impls, especially when all you want to do is quickly omit a single field which is `!Debug`.

This is implemented for enums, record structs, tuple structs, empty structs - and it sets us up to implement variations on this in the future for other traits (like `PartialEq` and `Hash`).

Thanks!

## Codegen diff
This is the difference in codegen for record structs with this patch:
```diff
struct Foo {
    bar: String,
}

impl fmt::Debug for Foo {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        todo!();
+        f.debug_struct("Foo").field("bar", &self.bar).finish()
    }
}
```

Co-authored-by: Irina Shestak <shestak.irina@gmail.com>
Co-authored-by: Yoshua Wuyts <yoshuawuyts@gmail.com>
Co-authored-by: Yoshua Wuyts <yoshuawuyts+github@gmail.com>

crates/ide/src/hover.rs

@@ -2732,8 +2732,8 @@ fn foo() {
                                     file_id: FileId(
                                         1,
                                     ),
-                                    full_range: 251..433,
-                                    focus_range: 290..296,
+                                    full_range: 252..434,
+                                    focus_range: 291..297,
                                     name: "Future",
                                     kind: Trait,
                                     description: "pub trait Future",

crates/ide_assists/src/handlers/replace_derive_with_manual_impl.rs

@@ -2,6 +2,8 @@ use hir::ModuleDef;
 use ide_db::helpers::{import_assets::NameToImport, mod_path_to_ast};
 use ide_db::items_locator;
 use itertools::Itertools;
+use syntax::ast::edit::AstNodeEdit;
+use syntax::ted;
 use syntax::{
     ast::{self, make, AstNode, NameOwner},
     SyntaxKind::{IDENT, WHITESPACE},
@@ -32,8 +34,8 @@ use crate::{
 // struct S;
 //
 // impl Debug for S {
-//     fn fmt(&self, f: &mut Formatter) -> Result<()> {
-//         ${0:todo!()}
+//     $0fn fmt(&self, f: &mut Formatter) -> Result<()> {
+//         f.debug_struct("S").finish()
 //     }
 // }
 // ```
@@ -111,7 +113,7 @@ fn add_assist(
         |builder| {
             let insert_pos = adt.syntax().text_range().end();
             let impl_def_with_items =
-                impl_def_from_trait(&ctx.sema, &annotated_name, trait_, trait_path);
+                impl_def_from_trait(&ctx.sema, adt, &annotated_name, trait_, trait_path);
             update_attribute(builder, input, &trait_name, attr);
             let trait_path = format!("{}", trait_path);
             match (ctx.config.snippet_cap, impl_def_with_items) {
@@ -149,6 +151,7 @@ fn add_assist(
 
 fn impl_def_from_trait(
     sema: &hir::Semantics<ide_db::RootDatabase>,
+    adt: &ast::Adt,
     annotated_name: &ast::Name,
     trait_: Option<hir::Trait>,
     trait_path: &ast::Path,
@@ -163,9 +166,116 @@ fn impl_def_from_trait(
         make::impl_trait(trait_path.clone(), make::ext::ident_path(&annotated_name.text()));
     let (impl_def, first_assoc_item) =
         add_trait_assoc_items_to_impl(sema, trait_items, trait_, impl_def, target_scope);
+
+    // Generate a default `impl` function body for the derived trait.
+    if let ast::AssocItem::Fn(ref func) = first_assoc_item {
+        let _ = gen_default_impl(func, trait_path, adt, annotated_name);
+    };
+
     Some((impl_def, first_assoc_item))
 }
 
+/// Generate custom trait bodies where possible.
+///
+/// Returns `Option` so that we can use `?` rather than `if let Some`. Returning
+/// `None` means that generating a custom trait body failed, and the body will remain
+/// as `todo!` instead.
+fn gen_default_impl(
+    func: &ast::Fn,
+    trait_path: &ast::Path,
+    adt: &ast::Adt,
+    annotated_name: &ast::Name,
+) -> Option<()> {
+    match trait_path.segment()?.name_ref()?.text().as_str() {
+        "Debug" => gen_debug_impl(adt, func, annotated_name),
+        _ => Some(()),
+    }
+}
+
+/// Generate a `Debug` impl based on the fields and members of the target type.
+fn gen_debug_impl(adt: &ast::Adt, func: &ast::Fn, annotated_name: &ast::Name) -> Option<()> {
+    match adt {
+        // `Debug` cannot be derived for unions, so no default impl can be provided.
+        ast::Adt::Union(_) => Some(()),
+
+        // => match self { Self::Variant => write!(f, "Variant") }
+        ast::Adt::Enum(enum_) => {
+            let list = enum_.variant_list()?;
+            let mut arms = vec![];
+            for variant in list.variants() {
+                let name = variant.name()?;
+                let left = make::ext::ident_path("Self");
+                let right = make::ext::ident_path(&format!("{}", name));
+                let variant_name = make::path_pat(make::path_concat(left, right));
+
+                let target = make::expr_path(make::ext::ident_path("f").into());
+                let fmt_string = make::expr_literal(&(format!("\"{}\"", name))).into();
+                let args = make::arg_list(vec![target, fmt_string]);
+                let macro_name = make::expr_path(make::ext::ident_path("write"));
+                let macro_call = make::expr_macro_call(macro_name, args);
+
+                arms.push(make::match_arm(Some(variant_name.into()), None, macro_call.into()));
+            }
+
+            let match_target = make::expr_path(make::ext::ident_path("self"));
+            let list = make::match_arm_list(arms).indent(ast::edit::IndentLevel(1));
+            let match_expr = make::expr_match(match_target, list);
+
+            let body = make::block_expr(None, Some(match_expr));
+            let body = body.indent(ast::edit::IndentLevel(1));
+            ted::replace(func.body()?.syntax(), body.clone_for_update().syntax());
+            Some(())
+        }
+
+        ast::Adt::Struct(strukt) => {
+            let name = format!("\"{}\"", annotated_name);
+            let args = make::arg_list(Some(make::expr_literal(&name).into()));
+            let target = make::expr_path(make::ext::ident_path("f"));
+
+            let expr = match strukt.field_list() {
+                // => f.debug_struct("Name").finish()
+                None => make::expr_method_call(target, make::name_ref("debug_struct"), args),
+
+                // => f.debug_struct("Name").field("foo", &self.foo).finish()
+                Some(ast::FieldList::RecordFieldList(field_list)) => {
+                    let method = make::name_ref("debug_struct");
+                    let mut expr = make::expr_method_call(target, method, args);
+                    for field in field_list.fields() {
+                        let name = field.name()?;
+                        let f_name = make::expr_literal(&(format!("\"{}\"", name))).into();
+                        let f_path = make::expr_path(make::ext::ident_path("self"));
+                        let f_path = make::expr_ref(f_path, false);
+                        let f_path = make::expr_field(f_path, &format!("{}", name)).into();
+                        let args = make::arg_list(vec![f_name, f_path]);
+                        expr = make::expr_method_call(expr, make::name_ref("field"), args);
+                    }
+                    expr
+                }
+
+                // => f.debug_tuple("Name").field(self.0).finish()
+                Some(ast::FieldList::TupleFieldList(field_list)) => {
+                    let method = make::name_ref("debug_tuple");
+                    let mut expr = make::expr_method_call(target, method, args);
+                    for (idx, _) in field_list.fields().enumerate() {
+                        let f_path = make::expr_path(make::ext::ident_path("self"));
+                        let f_path = make::expr_ref(f_path, false);
+                        let f_path = make::expr_field(f_path, &format!("{}", idx)).into();
+                        let method = make::name_ref("field");
+                        expr = make::expr_method_call(expr, method, make::arg_list(Some(f_path)));
+                    }
+                    expr
+                }
+            };
+
+            let method = make::name_ref("finish");
+            let expr = make::expr_method_call(expr, method, make::arg_list(None));
+            let body = make::block_expr(None, Some(expr)).indent(ast::edit::IndentLevel(1));
+            ted::replace(func.body()?.syntax(), body.clone_for_update().syntax());
+            Some(())
+        }
+    }
+}
+
 fn update_attribute(
     builder: &mut AssistBuilder,
     input: &ast::TokenTree,
@@ -207,41 +317,92 @@ mod tests {
     use super::*;
 
     #[test]
-    fn add_custom_impl_debug() {
+    fn add_custom_impl_debug_record_struct() {
         check_assist(
             replace_derive_with_manual_impl,
             r#"
-mod fmt {
-    pub struct Error;
-    pub type Result = Result<(), Error>;
-    pub struct Formatter<'a>;
-    pub trait Debug {
-        fn fmt(&self, f: &mut Formatter<'_>) -> Result;
-    }
-}
-
+//- minicore: fmt
 #[derive(Debu$0g)]
 struct Foo {
     bar: String,
 }
 "#,
             r#"
-mod fmt {
-    pub struct Error;
-    pub type Result = Result<(), Error>;
-    pub struct Formatter<'a>;
-    pub trait Debug {
-        fn fmt(&self, f: &mut Formatter<'_>) -> Result;
-    }
-}
-
 struct Foo {
     bar: String,
 }
 
-impl fmt::Debug for Foo {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        ${0:todo!()}
+impl core::fmt::Debug for Foo {
+    $0fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("Foo").field("bar", &self.bar).finish()
+    }
+}
+"#,
+        )
+    }
+    #[test]
+    fn add_custom_impl_debug_tuple_struct() {
+        check_assist(
+            replace_derive_with_manual_impl,
+            r#"
+//- minicore: fmt
+#[derive(Debu$0g)]
+struct Foo(String, usize);
+"#,
+            r#"struct Foo(String, usize);
+
+impl core::fmt::Debug for Foo {
+    $0fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_tuple("Foo").field(&self.0).field(&self.1).finish()
+    }
+}
+"#,
+        )
+    }
+    #[test]
+    fn add_custom_impl_debug_empty_struct() {
+        check_assist(
+            replace_derive_with_manual_impl,
+            r#"
+//- minicore: fmt
+#[derive(Debu$0g)]
+struct Foo;
+"#,
+            r#"
+struct Foo;
+
+impl core::fmt::Debug for Foo {
+    $0fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("Foo").finish()
+    }
+}
+"#,
+        )
+    }
+    #[test]
+    fn add_custom_impl_debug_enum() {
+        check_assist(
+            replace_derive_with_manual_impl,
+            r#"
+//- minicore: fmt
+#[derive(Debu$0g)]
+enum Foo {
+    Bar,
+    Baz,
+}
+"#,
+            r#"
+enum Foo {
+    Bar,
+    Baz,
+}
+
+impl core::fmt::Debug for Foo {
+    $0fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        match self {
+            Self::Bar => write!(f, "Bar"),
+            Self::Baz => write!(f, "Baz"),
+        }
     }
 }
 "#,

crates/ide_assists/src/tests/generated.rs

@@ -1385,8 +1385,8 @@ trait Debug { fn fmt(&self, f: &mut Formatter) -> Result<()>; }
 struct S;
 
 impl Debug for S {
-    fn fmt(&self, f: &mut Formatter) -> Result<()> {
-        ${0:todo!()}
+    $0fn fmt(&self, f: &mut Formatter) -> Result<()> {
+        f.debug_struct("S").finish()
     }
 }
 "#####,

crates/rust-analyzer/tests/slow-tests/tidy.rs

@@ -274,11 +274,13 @@ fn check_todo(path: &Path, text: &str) {
         "handlers/add_turbo_fish.rs",
         "handlers/generate_function.rs",
         "handlers/fill_match_arms.rs",
+        "handlers/replace_derive_with_manual_impl.rs",
         // To support generating `todo!()` in assists, we have `expr_todo()` in
         // `ast::make`.
         "ast/make.rs",
         // The documentation in string literals may contain anything for its own purposes
         "ide_db/src/helpers/generated_lints.rs",
+        "ide_assists/src/tests/generated.rs",
     ];
     if need_todo.iter().any(|p| path.ends_with(p)) {
         return;

crates/syntax/src/ast/make.rs

@@ -311,6 +311,9 @@ pub fn expr_method_call(
 ) -> ast::Expr {
     expr_from_text(&format!("{}.{}{}", receiver, method, arg_list))
 }
+pub fn expr_macro_call(f: ast::Expr, arg_list: ast::ArgList) -> ast::Expr {
+    expr_from_text(&format!("{}!{}", f, arg_list))
+}
 pub fn expr_ref(expr: ast::Expr, exclusive: bool) -> ast::Expr {
     expr_from_text(&if exclusive { format!("&mut {}", expr) } else { format!("&{}", expr) })
 }
@@ -318,6 +321,9 @@ pub fn expr_closure(pats: impl IntoIterator<Item = ast::Param>, expr: ast::Expr)
     let params = pats.into_iter().join(", ");
     expr_from_text(&format!("|{}| {}", params, expr))
 }
+pub fn expr_field(receiver: ast::Expr, field: &str) -> ast::Expr {
+    expr_from_text(&format!("{}.{}", receiver, field))
+}
 pub fn expr_paren(expr: ast::Expr) -> ast::Expr {
     expr_from_text(&format!("({})", expr))
 }

crates/test_utils/src/minicore.rs

@@ -31,6 +31,7 @@
 //!     eq: sized
 //!     ord: eq, option
 //!     derive:
+//!     fmt: result
 
 pub mod marker {
     // region:sized
@@ -334,6 +335,17 @@ pub mod cmp {
 }
 // endregion:eq
 
+// region:fmt
+pub mod fmt {
+    pub struct Error;
+    pub type Result = Result<(), Error>;
+    pub struct Formatter<'a>;
+    pub trait Debug {
+        fn fmt(&self, f: &mut Formatter<'_>) -> Result;
+    }
+}
+// endregion:fmt
+
 // region:slice
 pub mod slice {
     #[lang = "slice"]